Intelligent efficient servo-actuator for a downhole pulser

ABSTRACT

An improved energy efficient intelligent pulser driver used for generating a mud pulse in a MWD (measurement while drilling) application. In the pulser driver, a direct current (DC) powered control circuit activates a three-phase DC brushless motor that operates a servo-valve. Opening of the servo-valve equalizes pressure in a plenum causing the operation of a main valve reducing flow area and causing a pressure spike in the mud column. Closing of the servo-valve creates a reduction in mud pressure that operates the main valve and increases the flow area causing an end to the pressure spike. The servo-valve is powered both in opening and closing operations by the motor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/560,047, filed Apr. 6, 2004, and Canadian Application No. 2,463,354,filed Apr. 6, 2004, which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to a telemetry system, and inparticular to a measurement while drilling (MWD) system. Moreparticularly, the present invention relates to a servo-actuator for adownhole mud pulser for sending information from downhole to surface.

BACKGROUND OF THE INVENTION

The desirability and effectiveness of well logging systems whereinformation is sensed in the well hole and transmitted to the surfacethrough mud pulse telemetry has long been recognized. Mud pulsetelemetry systems provide the driller at the surface with means forquickly determining various kinds of downhole information, mostparticularly information about the location, orientation and directionof the drill string at the bottom of the well in a directional drillingoperation. During normal drilling operations, a continuous column of mudis circulating within the drill string from the surface of the well tothe drilling bit at the bottom of the well and then back to the surface.Mud pulse telemetry repeatedly restricts the flow of mud to propagatesignals through the mud upward to the surface, thereby providing a veryfast communication link between the drill bit and the surface. Dependingon the type of drilling fluid used, the velocity may vary betweenapproximately 3000 and 5000 feet per second.

A telemetry system may be lowered on a wireline located within the drillstring, but is usually formed as an integral part of a special drillcollar inserted into the drill string near the drilling bit. The basicoperational concept of mud pulse telemetry is to intermittently restrictthe flow of mud as it passes through a downhole telemetry valve, therebycreating a pressure pulse in the mud stream that travels to the surfaceof the well. The information sensed by instrumentation in the vicinityof the drilling bit is encoded into a digital formatted signal and istransmitted by instructions to pulse the mud by intermittently actuatingthe telemetry valve, which restricts the mud flow in the drill string,thereby transmitting pulses to the well surface where the pulses aredetected and transformed into electrical signals which can be decodedand processed to reveal transmitted information.

Representative examples of previous mud pulse telemetry systems may befound in U.S. Pat. Nos. 3,949,354; 3,958,217; 4,216,536; 4,401,134; and4,515,225.

Representative samples of mud pulse generators may be found in U.S. Pat.Nos. 4,386,422; 4,699,352; 5,103,420; and 5,787,052.

A telemetry system capable of performing the desired function withminimal control energy is desirable, since the systems are typicallypowered by finite-storage batteries. One such example is found in U.S.Pat. No. 5,333,686, which describes a mud pulser having a main valvebiased against a narrowed portion of the mud flowpath to restrict theflow of mud, with periodic actuation of the main valve to allow mud totemporarily flow freely within the flowpath. The main valve is actuatedby a pilot valve that can be moved with minimal force. The pilot valveadditionally provides for pressure equalization, thereby increasing thelife of downhole batteries.

Another example of an energy efficient mud pulser is described in U.S.Pat. No. 6,016,288, the mud pulser having a DC motor electricallypowered to drive a planetary gear which in turn powers a threaded driveshaft, mounted in a bearing assembly to rotate a ball nut lead screw.The rotating threaded shaft lifts the lead screw, which is attached tothe pilot valve.

Solenoid-type pulser actuators have also been used to actuate the mainpulser valve, however, there are many problems with such a system. Theuse of a spring to bias the solenoid requires the actuator (servo) valveto overcome the force of the spring (about 6 pounds) and of the mudprior to actuating the main valve. A typical solenoid driven actuatorvalve is capable of exerting only 11 pounds of pressure, leaving only 5pounds of pressure to actuate the pulser assembly. Under drillingconditions requiring higher than normal mud flow, the limited pressuresexerted by the solenoid may be unable to overcome both the pressure ofthe return spring and the increased pressure of the flowing mud,resulting in a failure to open the servo-valve, resulting in the mainvalve remaining in a position in which mud flow is not restricted, andtherefore failing to communicate useful information to the surface.

A further problem with the use of a solenoid to actuate the pulserassembly is the limited speed of response and recovery that is typicalof solenoid systems. Following application of a current to a solenoid,there is a recovery period during which the magnetic field decays to apoint at which it can be overcome by the force of the solenoid's ownreturn spring to close the servo-valve. This delay results in a maximumdata rate (pulse width) of approximately 0.8 seconds/pulse, limiting theapplication of the technology.

Moreover, the linear alignment of the solenoid must be exactly tuned(i.e. the magnetic shaft must be precisely positioned within the coil)in order to keep the actuator's power characteristics within a reliableoperating range. Therefore, inclusion of a solenoid within the tool addscomplexity to the process of assembling and repairing the pulseractuator, and impairs the overall operability and reliability of thesystem.

Existing tools are also prone to jamming due to accumulation of debris,reducing the range of motion of the pilot valve. Particularly whencombined with conditions of high mud flow, the power of the solenoid isunable to clear the jam, and the tool is rendered non-functional. Thetool must then be brought to the surface for service.

Stepper motors have been used in mud pulsing systems, specifically, innegative pulse systems (see for example U.S. Pat. No. 5,115,415). Theuse of a stepper motor to directly control the main pulse valve,however, requires a large amount of electrical power, possibly requiringa turbine generator to supply adequate power to operate the system forany length of time downhole.

Repair of previous pursers has been an as yet unresolved difficulty.Typically, the entire tool has been contained within one housing, makingaccess and replacement of small parts difficult and time-consuming.Furthermore, a bellows seal within the servo-poppet has typically beenthe only barrier between the mud flowing past the pilot valve's poppetand the pressurized oil contained within the servo-valve actuating tool,which is required to equalize the hydrostatic pressure of the downholemud with the tool's internal spaces. Therefore, in order to dissemblethe tool for repair, the bellows seal had to be removed, causing theintegrity of the pressurized oil chamber to be lost at each repair.

Furthermore, a key area of failure of MWD pulser drivers has been thefailure of the bellows seal around the servo-valve activating shaft,which separates the drilling mud from the internal oil. In existingsystems, the addition of a second seal is not feasible, particularly inservo-drivers in which the servo-valve is closed by a spring due to thelimited force which may be exerted by the spring, which is in turnlimited by the available force of the solenoid, and cannot overcome thefriction or drag of an additional static/dynamic linear seal.

It remains desirable within the art to provide a pulse generator thathas an energy efficiency sufficient to operate reliably and to adapt toa variety of hostile downhole conditions, has reduced susceptibility tojamming by debris, and is simpler to repair than previous systems.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at leastone disadvantage of previous mud pulsers and pulse generators.

In a first aspect, the present invention provides a downholemeasurement-while-drilling pulser actuator comprising a servo valvemovable between an open position which permits mud flow through aservo-orifice and a restricted position which restricts mud flow throughthe servo-orifice, the servo-valve powered to the open position andpowered to the closed position by a reversible electric motor.

In one embodiment, the servo valve includes a servo-poppet powered bythe motor in reciprocating linear movement towards and away from theservo-orifice.

In a further embodiment, the actuator may include a rotary to linearconversion system for converting rotary motion of the reversibleelectric motor into linear reciprocating movement of the poppet. Therotary to linear conversion system may include a threaded lead screwheld stationary and driven in rotation by a rotary motor. In thisembodiment, the lead screw may be threadably attached to a ball nut fromwhich the poppet depends, whereby the rotary motion of the motor causesrotation of the screw to result in driven linear movement of the ballnut and the poppet in either direction.

In a further embodiment, there is provided a servo-controller forcontrolling the powering of the servo-valve by the electric motor. Theservo-controller may further be capable of sensing the position of thepoppet with respect to the servo-orifice, such that the poppet positionis sensed when mud flow through the servo-orifice is restricted orunrestricted, and wherein the amount and direction of rotation of themotor from the sensed poppet position is counted and stored by thecontroller.

In another embodiment, the sensed position of the orifice restriction iscalibrated as the fully closed position of the poppet. The poppet'stravel is thereby monitored and controlled during operation to avoidunneeded collision or frictional wear between the poppet and theservo-orifice. The servo controller may sense the position of the poppetby sensing whether movement of the poppet is impeded, and theservo-controller counts the number of rotations of the motor until thepoppet is impeded and compares the number of rotations to an expectednumber of rotations to determine the position of the poppet with respectto the servo-orifice. The expected number of rotations can be preset toallow a predetermined rate of mud flow past the servo-orifice when thepoppet is moved away from the servo-orifice by the preset expectednumber of rotations.

In a still further embodiment, the servo-controller may include a debrisclearing command that is initiated when the number of rotations countedis not equal to the expected number of rotations. The debris clearingcommand may cause the motor to rapidly reciprocate the poppet todislodge any debris present between the poppet and the servo-orifice.

In another embodiment, the attachment between the poppet and the motorcomprises a dynamic seal to isolate the motor, rotary to linearconversion system and related drive components from the drilling mud inwhich the poppet and orifice are immersed when in operation.

In a further aspect, the present invention provides a method for causingthe generation of a mud pulse by a controlled pulser's main pulse valvecomprising the steps of: powering a pulser servo-valve in a firstdirection using a rotary motor such that mud is permitted to flow past aservo-orifice to activate a main mud pulse valve; and powering theservo-valve in a second direction using the rotary motor such that mudflow past the servo-orifice is restricted to deactivate the main mudpulse valve.

In one embodiment, the method further comprises the step of cuttingpower to the motor to hold the servo-valve in a particular positionwithin its range of motion to tailor the actuator's effect on the mainpulse valve and thereby tailor the pressure and duration characteristicsof a mud pulse.

In another aspect, the invention provides a servo-controller for usewith a downhole measurement-while-drilling pulser actuator, theservo-controller comprising a sensor, memory, control circuitry, and anoperator interface.

In one embodiment, the sensor is a mudflow sensor, pressure sensor,temperature sensor, rotation-step counter, position sensor, velocitysensor, current level sensor, battery voltage sensor, timer, or an errormonitor.

In another embodiment, the memory stores time-stamped or counted sensedevents together with an event-type indication. The servo-controller maybe programmable to cause an action within the actuator responsive to asensed event, a time, an elapsed time, a series of sensed events, or anycombination thereof.

In a further embodiment, the user interface provides information frommemory to the operator, and may allow an operator to alter theprogramming of the control circuitry. Other aspects and features of thepresent invention will become apparent to those ordinarily skilled inthe art upon review of the following description of specific embodimentsof the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIGS. 1A and B are a longitudinal cross sectional view of the upper andlower portions of an embodiment of the mud pulser during mud flowthrough the servo orifice; and

FIGS. 2A and 2B are a longitudinal cross sectional view of the upper andlower portions of an embodiment of the mud pulser during mud flowrestriction by the poppet.

DETAILED DESCRIPTION

The present invention relates to an apparatus and method for actuating amud pulser telemetry system used during well-drilling operations. Thepresent apparatus allows a servo-valve to be powered both in opening andclosing to activate a main mud pulser valve, and does not rely on asolenoid system. The powered opening and closing of the servo-valveresults in various functional and economic advantages, including theability to clear debris from the restricted portion of the mud flowpath,and faster data rates due to elimination of inherent operating delays inthe solenoid systems of previous tools, with the end result of providinga pulser driver which consumes a minimal amount of DC power whileproviding more force with which to drive the servo-valve's poppet ineach direction. Therefore, the actuator remains functional at acomprehensive range of downhole drilling conditions.

Furthermore, in the embodiment shown in the Figures, the present deviceis designed to have several independent, interconnected housings, andemploys a double seal between the oil compartment and the drilling mud,which simplifies assembly and repair of the tool. Theassembly/disassembly is simplified to reduce repair turnaround time byusing modular components.

Additionally, the use of a stepper motor, electric load sensors, andcontrol circuitry in a powered-both-directions servo-valve system willallow for self-calibration of the tool and self-diagnosis and errorcorrection unavailable in other systems. In an embodiment of theinvention, as shown in FIGS. 1A and 1B, a three-phase stepper rotarymotor 1 is monitored and controlled by a servo-controller 10, the rotarymovement of the motor 1 being converted into linear movement of a poppet21, thereby opening and closing a servo-valve 20 to actuate a mud pulsermain valve (not shown). Communication of information to the well surfaceis accomplished by encoded signals, which are translated to producepressure surges in the downward flow of the pressurized mud. It isrecognized that although the drilling fluid is generally referred to asmud, other drilling fluids are also suitable for use with the presentinvention, as is well known in the art.

With reference to the Figures, the mud pulser actuator is lowereddownhole and, in the embodiment shown, generally includes a plurality ofserially interconnected housings 2, 3, 4, 5, 6, 7, and 8, an electricalconnector 9, a servo-controller 10 for controlling the operation of arotary motor 1, and a servo-valve assembly 20 that is driven in linearmotion by the rotary motor 1. The servo-valve assembly includes a poppet21 capable of linear reciprocating movement to and from a seal surface22 of a servo orifice 23, thereby opening and closing the servo orifice23 to allow or prevent the passage of pressurized mud and therebyactuate a pulser (not shown, connected to the lower end 2 a of thelowermost housing 2) to generate a pressure pulse for telemetricpurposes.

Mechanical System

A rotary-to-linear coupling system 30 a, 30 b (hereinafter referred toas coupling system 30) is used to translate the torque from the rotarymotor 1 into linear movement of the servo-valve shaft 24, which ispreferably a series of connected shafts for transferring linear movementfrom the coupling system 30 to the servo poppet 21. Preferably, theservo shaft includes a spline shaft 24 a, which passes through a splinecoupling 24 b that can be used to prevent rotation of the shaft 24 awhen necessary. The coupling system 30 also includes seals which serveto isolate the rotating mechanism from the downhole mud.

In the embodiment pictured in FIGS. 1A and 1B, the rotary motor 1, iselectrically powered through an electrical connection 9, by a powersource (not shown). When activated, the motor 1 rotates a lead screw 31that is mounted within a bearing support 32, causing a ball nut 33 tomove threadably along the lead screw 31. Linear movement of the ball nut33 results in dependent linear movement of the servo shaft 24, and servopoppet 21. When driven in the forward direction, the rotary motor 1 willcause linear movement of the poppet 21 away from the servo-valve seat22, to allow passage of pressurized mud through the servo-orifice 23 toactivate the main mud pulser valve to close. When the motor 1 drives thelead screw 31 in the reverse direction, poppet 21 is urged towards theseal surface 22 to cover the servo orifice 23, as shown in FIG. 2B, andmud is therefore prevented from passing through the servo orifice 23 toactuate the mud pulser main valve to open.

The spline shaft 24 is surrounded by lubricating fluid, which must bepressurized against the downhole hydrostatic pressure. As shown, apressure compensator in the form of a membrane or bellows 42 allowsreservoir fluid to substantially equalize the pressure via a part 43.The pressure compensator be a membrane, bellows, piston type or othertype known in the industry. In addition to a bellows seal 40, anadditional seal 41 may be added to hold oil inside the chamber of thetool, with the bellows seal 40 preventing mud from reaching theadditional seal 41. The dual seal 40, 41 maintains the integrity of thelubrication chamber during operation and during replacement of thebellows seal 40 during maintenance. The addition of this seal 41 doesnot negatively impact performance of the actuator due to the improvedpower characteristics of the system, as will be discussed below.

In a preferred embodiment, the construction of the device allows mostdownhole clogs, where debris in the mud may stop the poppet 21 fromsealing with the seal surface 22, to be easily cleared as will bedescribed below, and the serially interconnected housing design allowssimple and rapid repair of the tool when necessary.

The valve assembly 20 is preferably composed of a wear resistantmaterial such as tungsten carbide or ceramic to maximize the efficiencyof the tool and to minimize maintenance of the tool, and is preferablyreplaceable.

Operation

When restriction of mud flow by the main valve is desired, the rotarymotor 1 will be activated by the servo-controller 10 in the forwarddirection. As shown in FIG. 1B, forward powering of the rotary motor 1will cause the lead screw 31 to turn in the forward (for example,clockwise) direction, thereby raising the ball nut 33 and lifting theservo poppet 21 from the servo-valve seat 22. This will allow mud flowto pass unrestricted through the servo-orifice 23 to actuate the mainmud pulse valve, restricting mud flow to generate a pulse that istransmitted to the surface. The current-consuming portion of the circuitis then shut down until a further signal is received from theservo-controller 10. The lack of current to the motor 1 results in themotor 1 being immovable and therefore acting as a brake to preventfurther movement of the poppet 21 until further activation of the motor1.

Subsequently, when the servo-controller 10 initiates reverse motion bythe motor 1, the lead screw 31 is rotated in the reverse direction (inthe example, counterclockwise) by the motor 1, causing the ball nut 33and servo shaft 24 to move towards the servo-valve seat 22 as shown inFIG. 2B. Closure of the servo-valve 20 causes opening of the main mudpulser valve to allow mud to flow unrestricted to the surface. Thecurrent-consuming portion of the circuit is then shut down until afurther signal is received from the servo-controller 10. The motor againacts as a brake until further power is applied (by shorting its coilstogether).

The lead screw 31 and ball nut 33 may be replaced by an alternate systemof rotary to linear conversion, however a lead screw 31 and ball nut 33are advantageous as they are relatively small in size and may beprovided with bearings to provide a low-friction mechanism with highload capacity, durability, and low backlash tolerance. The lead screw 31may be held in contact with the motor 1 by a bearing support 32 or anyother suitable means.

The presently described system of using a stepper motor 1 to drive aservo-valve has several advantages. The powering of the servo-valve 20in both directions allows greater direct control of the servo-valve 20,avoids the previous necessity of using a return spring in the servoassembly, and therefore the energy required is similar to that of theforce of the downhole mud flow. This results in an energy efficientsystem, and results to date indicate that the presently described systemcan supply a force of 100 pounds of pressure for less energy thanprevious systems, particularly than those which employ a solenoidactivator. Thus, the present system can overcome higher pressures on thepoppet valve 21, allowing the system to clear itself of debris, andpermitting use in a wide range of downhole conditions, includingconditions of higher pressure and higher volume mud flow, and inconditions when the mud is contaminated or is very dense.

Use of a rotary motor powering the servo-valve in both directions alsoallows the system to be more responsive than solenoid systems, resultingin a faster data rate with more accurate or precise pulse-edge timing.Experimental results indicate that data rates of 0.25 seconds/pulse arepossible with this system, as compared to 0.8 to 1.5 seconds/pulse insolenoid systems.

Flow Detection & Diagnostic Software

The servo controller detects the position of the poppet 21 against theservo-valve seal 22 by counting the number of rotations made by themotor until further movement of the poppet is impeded. For example, ifthe poppet 21 is generally programmed to attain an unseated positionthat is three forward motor rotations away from the seated position,upon seating activation by the servo-controller 10, the motor will turnthree reverse rotations, at which point further rotation will be impededdue to seating of the poppet 21 on the seal 23. On unseating activationby the servo controller 10, the motor will turn three complete forwardrotations to return the poppet to its pre-programmed unseated position.Seating can be sensed by an increase in current drawn by the motor, fromwhich a large opposing force (like stopped motion due to valve seating)is inferred. The control circuitry also senses rotation of the motorsand can count rotations and direction of rotation.

Debris may enter the device with the mud, potentially causing jamming ofthe poppet. The servo controller 10 can be programmed to detect andclear jams from the servo-valve 20. For example, debris may becomelodged at the servo-valve seal 22, preventing the poppet from fullysealing against the valve seal 22. In such a situation, the motor wouldbe prevented from completing its three reverse rotations. This is sensedby the servo-controller 10, which will then attempt to dislodge thedebris. The dislodging sequence may include rapid reciprocation of thepoppet 21 towards and away from the seal 22, or may include furtherreverse rotations on the subsequent reverse rotation. For example, ifthe motor was able to turn only two reverse rotations, theservo-controller 10 will recognize that the valve did not properlyclose, and will adjust one or more subsequent forward and/or reverserotations to ensure that the poppet 21 is able to seat against the valveseal 22. Similarly, debris may cause the poppet to not fully open,resulting in appropriate corrective action by the servo-controller onthe next motor 1 activation. In either case, a processor provides areport of measurements recorded and controls the following cycle of thebrushless motor's rotation accordingly.

The ability to detect and clear most jams within the tool allows a morerobust design of the tool in other respects. For example, as the toolcan easily clear particulate matter from the servo-valve assembly, thetool can be provided with larger and fewer mud ports, and may includereduced amounts of screening. Screening is susceptible to clogging, andso reducing screening leads to longer mean time between operationfailure of the device in-hole; and will reduce the velocity of any mudflow through the tool, reducing wear on the bladder and other parts.Further, the removal of several previously necessary components (such asthe return spring, transformer, and solenoid and related electronics)contributes to a tool of smaller size (in both length and diameter) thatis more versatile in a variety of situations. For example, embodimentswith outside diameter less than 1⅜″ (approaching 1″) or length less thanfour feet have been achieved, although these dimensions are not by wayof limitation, but by example only.

Custom software also has the ability to track downhole conditions, andalso uses a sensor to detect mudflow. When mudflow is detected, a signalis sent to the Directional Module Unit (not shown), to activate theoverall system. The system also has the ability to time stamp eventssuch as start or end of mudflow, incomplete cycles or system errors, lowvoltages, current, and the like, as well as accumulated run-time, numberof pulses, number of errors, running totals of rotations or motorpulses. Wires or conductors may also be easily passed by the pulsersection to service additional near-bit sensors or other devices. Thesoftware that detects the mudflow can be configured for different timedelays to enable it to operate under a larger variety of downholedrilling conditions than its predecessors. The mudflow detectioncapability can also be used to calibrate or confirm the closed positionof the poppet.

In addition, a user may monitor such data as well as any downholesensors using a user interface attachable to the tool. Such sensors mayinclude pressure or temperature sensors, rotation step-counters, travelor depth sensors, current levels, battery voltage, or timers. The usercould monitor each component of the actuator to determine when the toolmust be removed from downhole for repair. A user may, in turn, programan activity to cause an action or correction in response to a sensedevent.

The above-described embodiments of the present invention are intended tobe examples only. Alterations, modifications and variations may beeffected to the particular embodiments by those of skill in the artwithout departing from the scope of the invention, which is definedsolely by the claims appended hereto.

1-15. (canceled)
 16. A servo-controller for use with a downholemeasurement-while-drilling pulser actuator, the servo-controllercomprising a sensor, memory, control circuitry, and an operatorinterface.
 17. The servo-controller of claim 16 wherein the sensor is amudflow sensor, pressure sensor, temperature sensor, rotation-stepcounter, position sensor, velocity sensor, current level sensor, batteryvoltage sensor, timer, or error monitor.
 18. The servo-controller ofclaim 16 wherein the memory stores time-stamped or counted sensed eventstogether with an event-type indication.
 19. The servo-controller ofclaim 16 wherein the control circuitry is programmable to cause anaction within the actuator responsive to a sensed event, a time, anelapsed time, a series of sensed events, or any combination thereof. 20.The servo-controller of claim 16 wherein the operator interface providesinformation from memory to the operator.
 21. The servo-controller ofclaim 16 wherein the operator interface allows an operator to alter theprogramming of the control circuitry.